Impedance-modifying device



Feb. 19, 1929.

IMPEDANCE MODIFYING DEVICE LEWIS 1 t w a 4 s t I e e h a S 2 W W 6 a I Rw w v M m l k c M y u 7 m n a 2 M m c k e C D W d I M e I l m w n M W Iw M w M m m m m M m M M M m m w "5* initial:

INVENTOR flfil A TTORNE Y Feb. 19, 1929.

1,702,305 B. F. LEWIS IMPEDANCE MODIFYING DEVICE Filed Dec. 27, 1926 2Sneets-$heet 2 w seam 0 bad!!! 0.94%.; 13 e 5 non bade at f7 1N VENTOR RLewis A TTORNEV Patented eb. 19, 1929.

UNITED STATES PATENT OFFICE.

BENJAMIN F. LEWIS, OF BROOKLYN, NEW YORK, ASSIGNOR TO AMERICAN TELEPHONE AND TELEGRAPH COMPANY, A CORPORATION OF NEW YORK.

IMPEDANCE-MODIFYING DEVICE.

Application filed December 27, 1926. Serial No. 157,269. I

This invention relates to the application of impedance modifying devicesto lines transmitting a voice-frequency telephone system andhigh-frequency carrier telephone or can rier telegraph system, andparticularly to transmission lines composed of a plurality of linesections having different electrical characteristics.

An illustration of one type of transmission line to which theapplication of impedance modifying means in accordance with this invention would be desirablewould be a line, composed of two sections ofnon-loaded open wire circuit inter-connected by a non-loaded cablecircuit, and transmitting a repeatcred voice frequency system, involvingfrequencies, say from about 300 to 3,000 cycles, and one or more carriersystems. involving frequencies from about 3,000 cycles upward to about30,000 cycles, depending on the number and type of carrier systems. Dueto the differing electrical characteristics or impedances of the cableand open wire circuits, considerable transmission loss results atcarrier frequencies due to'the reflection effects introdueed by theirregularity at the junction points of the incidental cable circuit andthe open wire circuits. This reflection loss at carrier frequenciesmight be reduced to a considerable extent by the insertion at the endsof the incidental cable circuit of impedance modifying transformers eachhaving an im pedance ratio equal to the ratio between the open wirecircuit and cable circuit character istic impcdances at the carrierfrequencies. However, the use of transformers which will accomplish thispurpose is limited by the fact that their insertion in the transmissionline prbduces an impedance irregularity in the line at voice frequencieswhich will tend to materially increase the unbalance of the voicefrequency repeaters in the line. Accordingly, the present inventionprovides a method of and arrangements for so modifying the impedance ina transmission line of theabove type that the reflection losses atcarrier frequencies may bereduced without any mate- 'rial detrimentaleffect on the balance of the stood from the following descriptiontogether with the accompanying drawing in the Figures 1 to 11 of whichthe invention and the principles of its operation are illustrated. Fig.1 illustrates the variation with frequency of the ratio of thecharacteristic impedance of a typical non-loaded open wire line to thecharacteristic impedance of a typical nonloaded cable. Fig. 2 is aschematic showing of a cable circuit interconnecting two open wirecircuits. Fig.3 is the equivalent of Fig. 2. F1gs. 4 and 5 are schematicshowings of connections between a single open wire cir cuit and a cablecircuit. Figs. 6, 7, and 8 are schematic showings of connections betweentwo open wire circuits and a cable circuit. Figs. 9, 10, and 11 areschematic showings of practical embodiments of the invention.

In Fig. 2 is illustrated a transmission line composed of two non-loadedopen wire line sections 2 and 3 interconnected by a non-loadedcablecircuit of relatively low impedance as compared to the impedance ofthe open wire line. These sections are interconnected by theauto-transformers 4 and 5 which have at their midpoints the condensers 6and 7. so As has been previously pointed out, in a trans mission line ofthe above mentioned-type the reflection loss at carrier frequencies maybe reduced to a considerable extent by the insertion at the ends of thecable of the impedance s5 modifying transformers such as 4; and 5, eachhaving an impedanceratio equal to the ratio between the open wirecircuitand cable circuit characteristic impedances at carrier frequencies.However, in the voice frequency range this impedance ratio variesconsiderably from the value for the carrier frequency range both inmodulus and in angle. This variation is illustratedgraphically by thecurves of Fig. I.

In order to understand what these curves signify, a few words ofexplanation will be given as to What is meant by the modulus and angleof the impedance of a circuit. As

.is well known, the impedance of a circuit has two components,resistance and reactance. If these components be represented bya vector'diagram, in which the vector representing the magnitude of therca'ctance is drawn at right angle to the vector representi'no' theresistance, these vectors may be reso Yeti into an equivalent vectordrawn at an angle to the resistance vector. The length or magnitude ofthe equivalent vector is known as the modulus of the impedance.

. dividing themodulus of one circuit by that of the other to obtain themodulus of the The angle between the equivalent'vector and -theresistancecomponent is known as the angle of the'impedance. If now wemeasure the reactance and resistance components of each of two circuitsat a given frequency, the ratio of the impedance of the two circuits maybe expressed in terms of the impedance moduli of the. two circuits andthe impedance angles of the two circuits. This is done by that at'theupper carrier frequencies. illus trated, such as at 30,000 cyclesfltheimpedance ratio has a modulus of about 4.6 with a 6 angle, while at thelow voice frequencies, such as 300 cycles, the impedance ratio has amodulus of only lfibut with a 25 angle. Due to this variation inimpedance ratio'the use of a t-I'ZIHSfOIIDGl' having the impedanceratioof the circuits at carrier frequencies would increase the impedanceirregularity at voice frequencies at the 1unct-1on of a nonloaded cablecircuit and a non-loaded open wire circuit to such an extent that itwould be very objectionable from the repeater balance standpoint whenthe circuit is used to transmit. a repeatered voice frequency system.

i If the incidental cable circuit is electrically short at voicefrequencies and is equipped at each end with properlydesignedtransformers, this variation in theratio between the impedancesof the open wire circuit and cable circuit may be unimportant, since the1 length of the cable makes its effect small,

and the transformers work ractically back to back. Assuming the use 0two equal ideal transformers, i. e., transformers which have iguaaou erscause impedance distortion effects which must be taken intoconsideration. These ef fects consist of an addition to the resistanceand 'reactivecomponent to the impedance at voice frequencies lookinginto the high side of one .transforn'ler. Thus, in the practical case,under theordinary assumptions made above, there results an impedanceirregularity at this point which. may be great enough to make the use ofthe transformer mentioned very objectionable on voice frequencyrepeatered circuits.

Accordingly, it will be seen that the use of impedance modifyingtransformers to correct for reflection losses at carrier frequencies maybe objectionable from the transmission standpoint, with respect to voicefre quency repeatered channels for two reasons,-first,'because' of thevariation with frequency of the impedance ratio of the two circuits, andsecondly, because of the impedance irregularities.introduced by theinherent impedance distortion effect of the transformers themselves. Itis these two objectionable features to the use oftransformer's that thepresent arrangement and application of transformers in the transmissionline will substantially overcome, as will appear more fully hereinafter.

The effect of the inherent impedance distort-ion of the transformer atvoice frequencies may be more fully understood with rcfcrence to Fig. 3which is a schematical showing of the two line sections 2 and 3interconnected through the transformers L and 5 and the cable circuit 1;The transformers have'bcen represented byequ-ivalent T networks ofimpedances having the series components Z and Z These components Z and Zare equivalent to the copper resistance of the coils and the leakagereactance. Z and Z represent the transformer mutual impedance. Atcarrier frequencies this mutual impedance factor would be very large.However, at

voice frequencies, it might be small enough to be comparable to the lineimpedances which are termed Z and Z Let it be assumed that theauto-transformers have equal ratios at yoice frequencies of 5:1, thatthe line impedance,'such as Z, or Z is equal to 050 Ill] ohms, that Z,and Z; are negligibly small and equal to +y' 3,000, this being a valueconsistthat the value-of Z at voice frequencies is ent with good designfor the conditions noted.

Thil equivalent to W=GSQLZ The impedance at points cd, or

Z a5/ i ag+5=127/ 1ag Z as seen through the transformer at A=127Q235=G35 2 The impedance at points ab or 5 Hence it will be seen that asimpedance since the impedance of the condenser is neg- Z1 =597+j 2 :37and as the line impedance ligihle at high frequencies. The diflierenceis Z 5 650, that there is a considerable irreguquite material atvoicefrequencies, however, larity between these impedances. V Accordandthe larger the impedance of the coningly, it will be seen that at voicefrequencies denser, the larger will be its effect in chang- 0 theinherentdistortion effect of the transing theeliect-ive transformerratio, particutorinei's assuming well designed transformlarly at voicefrequencies. For a trans crs of equal ratio, will introduce anobjection-' former having a high side inductance'of 3 able impedanceirregularity in the line. henries, a 2 mt. condenser would raise Inactual practice the auto-transformers, as the impedance ratio from 'anominal value 15 shown inFig. 2, will have condensers at their of 5 to 1to an effective ratio of about midpoints for circuit operationreasons,-i.e, 6 to 1. Similarly, a 3 to, 1 nominal ratio to prevent theexistence 0% a direct-current would be raised to about 3.5 to 1, Theinshjunt across the line which would prevent sertion of a midpointcondenser also affects superposed telegraph operation by the usual theseries arms of the transformer equivalent 2e composite method. Thesecondensers would network so that besides representing the normally be ofequal values in both transcopper loss and the leakage impedance, theseformers and would ordinarily have a capacarms also represent some of thecondenser ity of? Int. or lower, in order to keep cross reactance. firebetween superposed telegraph circuits H t v i fr i taking into 25 low.The cfiect o inse ngafi h consideration all the impedance componentsmidpoints of the windings of the auto-transf th t -t f Z i bi formers isto increase the effective transli with Z nd z im d h former ratio abovethe nominal ratio at voice i g a magnitude of about 875. With a 5 1frequencies. Th s i pp r more fully nominal ratio, which. as has beenseen, is.

so from the following discussion, together with 7 i l t t an eff ctiveratio of 6 1 Figs. at and 5. For an auto-transformer, as z zlgtij Ishown in Fig. 4, by suitably proportioning Z seen through transformer atA=ST5o the windings, the nominal impedance ratio Z i bi gfi Z d Z=1()()() would he made the same the ratio of the Z =10()() 1 Iimpedances ofthe lines which it interconnects, It ill b seen th t h Z ip d since this is the COllllltlOll 01 maximum 81]- with thg ling inpedance ZL; that the natch ergy transfer. In the case illustrated, thei 'very poor. It, in fact, amounts to an v r n in'ncdagnce irre ularit'introduced bv the 85 nominal impedance r tio would be lhc tmlnsformers0%7zVhiCh would m'ake it 40 transformer ratio would, as a first degreeapexceedingly dillicult to balance voice freproximation. be independentof frequency. quency repeater apparatus in the line.

Now, after setting the transformer ratio :15 At carrier frequencies the'cflcct ot' the transformer. is that of a shunt capacity rei l ll acondenser is added at midpoint, as

$6,, actance and a series inductance; -.The shuntillustrated in Fig. 5,the effective impedance ing impedance of the capacity rcactance 1:3

ratio becomes generally high enough for the range of fre- Z +Z quenciesinvolved so that at carrier frcquen m tires, the auto-transtormcrs areequivalent to a T network having series arms of small im- 50 Thiseffective ratio is not materially greater pcdance and a shunt arm havingan 1mthan the nominal ratio at carrier frequencles, pedance winch 1svery large as compared with the impedance of the open wire line.

The resultant impedance equivalent to the shunt arm in parallel to theline impedance Withthe'series arms, will therefore not be greatlydifferent from the impedance of the open wire line, which at carrierfrequencies may be assumed to be 600 ohms. Hence, at carrier frequenciesof 30,000 cycles we have from Fig. 3 j

Z =120w+(tl10 effective ratio being substantially the same atcarrierfrequencies as the nominal ratio of 5:1)

Z seen through transformer at A 60001 Obviously in view of the matchingof impedances' Z and Z there would be only a small reflection loss atcarrier frequencies under these conditions.

Owing to the had irregularity at voice frequencies, it might bedesirable in accordance with this invention to change the ratio of thetransformer at A to make the impedance Z a substantial match to theimpedance Z at voice frequencies. This would, of course, causereflection loss at the carrier frequency. For purposes of illustration,let us change the nominal ratio of the transformer at A denser, theeffective ratio, as has been pointed out under such conditions, would become'3.5:1. Under these conditions ratio 3.521=510w V.

Z". ,,Z Z in combination with 510w 850w i Compared to the line impedanceZ it will be seen that this'gives an impedanceirregularity of only 33%at voice frequencies in stead of %for the case previously pointed out. 7While this impedance irregularity of 33% is greater than would bedesired at voice frequencies, it may be reduced by further change in thenominal transformer ratio. Before considering this possibility, -let ussee what the use'of a 3:1 nominal ratio for the transformer at A does atcarrier frequencies.

As previously explained, the effective ratiowith a 2 mf. midpointcondenser will be up proximately 3 1 at carrier frequencies. i ls hasbeenshown previously at carrier fre-- quencies Z p 120 o+ Z at 3:1ralioseen through transformer ill) .A 360(o+ i As Z at carrier frequencies isvery high as compared to 360w This loss involved by the use of a 3 1 anda 5:1 nominal ratio at A and B, respectively, would therefore'beallowable at carrier frequencies. Hence, we might further reduce mately3.

from 5:1 to 3: 1. Due to the midpoint con' Z seen through transformer-atA with treason impedance ratio at voice frequencies with out reducingthe ratio at carrier frequencies.

1 This is evident from the fact previously noted that the nearer Zapproaches zero, the nearer will theefl'ective ratio at voicefrequencies approach the nominal ratio.

' Hence, if weincrease the size of the midpoint capacities (tlmsdecreasing their impedance) we may'reduce the effective ratio at voicefrequencies without reducing the effective ratio of the transformer atcarrier frequencies. For example, if We use a 3 mi? condenser at themidpoint of the" transformer at A, its effective ratio at 300 cycleswill be approxi- Fig. 3,

Z seen through transformer at A with ratio of 3.2 to 1=4l60w Z wZ and Zin combination with l60w=750w A It will be seen that, as compared withthe line impedance, this results in an irregularity at'voice frequenciesof only 19%. The irregularity at carrier frequencies is'not materiallychanged. It will therefore be apparent that i in accordance with thisinvention by reducing the nominal ratio of the transformer and byadjusting itsmidpoint capacity, the irregularity at; voicefrequenciesmay be brought to an allowable value at the expense of someincrease in the reflection loss at carrier frequencies as compared withthe ideal condi- 'tion involving an exact matching of impedanees atcarrier frequencies.

In order to more clearly understand how the impedance" irregularities atvoice frequencies may" be .brougl'it Within allowable limits at only asmall increase in the reflection loss at carrier frequencies inaccordance with the principles of this invention, there is j presented aseries ofcalculations based on conditions met in actual practice. Thesemay more readily .beunde rstood from reference to Figs. 6, 7 and 8. q

In Fig. (tare shown two line sections 2 and interconnected by the cablel'without the use of autotransformers. Line sections 2 -'and 3 arenon-loaded 165 mil open wire cir- 2 to 1. Now referring again to are 2mi". each.

due the cable is about the same as the cable equivalent, which is aboutA T. U. Accordingly, the presence of the cable introduces anirregularity at voice frequencies of about 28% which in this particularcaseis allowable.

In Fig. 7 auto-transformers have been inserted at the junction points ofthe cable and the open wire circuits. These transformers have the samenominal ratio of 5:1. The voice frequency case may be considered firstwith a frequency of 300 cycles. The line impedance Z =635 190. If thetransformers are ideal, then Z =Z and the impedance irregularity ataZ)=O. However if real transformers used in "practice are utilized, thenZ =950- 490, and the impedance irregularity at ab ht carrier frequenciesof 25 kilocycles, due

formers is 2 T. U. Therefore the net ain at carrier frequencies is 82==6 T. U.

In Fig. 8 the transformers at the junction points of the cable and openWire circuits have inserted at their midpoints the con densers C and CLet it be assumed that the values of these condensers are the same andThe transformers have different ratios, the transformer at ab having a 31 ratio and the other transformer having a 5: 1 ratio. Under theseconditions at a voice frequency of 300 cycles Z 750-j 380 and theimpedance irregularity at ab=33%. This irregularity is slightly higherthan thevalue 28 given above as allowable. As has been previouslypointed out in the theoretical case the impedance irregularity at ab canbe further reduced by further lowering of the nominal ratio of thetransforn'ier at of). Doing this, however, would sacrifice transmissionat carrier frequencies, since the reflection loss would be increased atsuch a point. As has been previously pointed out, the same effect atvoice frequencies may be accomplished by increasing the capacity of thecondenser at the midpoint of the transformer at ab. This would notsacrifice any transmission at carrier frequencies, since the effect ofthe condenser is negligible in that range. Accordingly, let us changethe value to 3 inf. Then Z =700; 300, and the impedance irregularity atab=19%. At carrier frequencies the loss due to the reduction of thenominal ratio of the transformer at a?) from 5 :1 to 3 1 is only about.2 T. U.

Hence by applying the principles of this inventionto the above discussedcase, we find that the net gain at carrier frequencies due to thetransformers will be about 5.8 T. U. ()i' in other words, the total lossdue to the The utilization in accordance with the principles of thisinvention of transformer units having different ratios at the ends of anon-loaded cable circuitas applicable in actual telephone practice incases where the impedance irregularity at voice frequencies is importantfrom thevoice frequency repeater standpoint at only one end of thecable. In Figs. 9, 10 and 11 are illustrated three different serviceconditions found in actual practice in transmission lines to which thetransformer units of this invention are applicable.

In Fig. 9 are shown two open wire circuits interconnected by a cablewith transformers at A and B,'and only one voice frequency repeaterlocated at R, no voice frequency repeater being involved beyond B. Itwill'be seen that the impedance 1rregularity at voice frequencies willbe important only at the end of the cable circuit at A, and hence theratio ofthe transformer thereat will be adjusted in i accordance withthfs invention to correct for this irregularity. There would be no voicefrequency repeater apparatus in the open wire circuit connected to thecable at B and hence any impedance irregularity at B would beunimportant. In other words, the invention is applicable when thedistance R-A is short enough so that the impedanceirregularity atvoicefrequencies at A due to correcting for reflection losses at carrierfrequencies is great enough'to effect the repeater balance at R.

In. Figure 10 is shown a line in which voice frequency repeaterapparatus is included in both of the open Wire lines, as at R and R. Therepeater at R is quite close to the cable, and is effected by impedanceirregularities at A. Hence the different ratio transformer unit of thisinvention may be utilized to correct for the eflcctof this impedanceirregularity at A on the repeater at It. The irregularity at I3 will notaffect the repeater at R as it will be located at such a distance fromBthat the effeet of the irregularity introduced at B will be small at Rdue to the attenuation of the line BR.

In Fig. 11 is illustrated a line having a repeater at It close enough toA to be affected by irregularities thereat and also having a repeater atR close enough to B to be affected by irregularities thereat. Underthese conditions the transformer unit of this invention may be utilizedto correct for the effect on R of irregularities at A as heretofore. Tocorrect for the effect of irregularities at B on R the repeater at Rwould have a balancing nettion may be embodied in many other formsandcircuits without departing from the spirit of the appended claims.

What is claimed is:

1. A transmission line over which currents of carrier frequenciesandcurrents of voice frequencies are transmitted, said transmission linecomprising two open wire circuits of relatively high impedanceinterconnected by a cable circuit of relatively low impedance, balancedvoice frequency repeater apparatus in one of said open Wire circuits,and impedance modifying means at the junction points of said cable andopen wire'circuits, said impedance modifying means comprisingautotransformers having condensers at the midpoints of their windings,the constants of each of said impedance modifying means being soadjusted with respect to the character istics of said individual openwire circuits, which differ due to the inclusion of said balancedrepeater apparatus in only one thereof, that the variation in impedancedistortion at different transmitted frequencies introduced mossesthereby in said line will be reduced to a minimum.

2. A transmission line over which currents of carrier frequencies andcurrents of voice frequencies are transmitted, said transmission linecomprising two open wire circuits of relatively high impedanceinterconnected by a cable circuit of relatively low impedance, voicefrequency repeater apparatus in one of said open wire circuits, andimpedance moditying means at the junction points of said cable and openwire circuits, said impedance modifying means comprisingauto-transformers having condensers at the midpoints of thelr windings,the auto -trans'lo rmer lnterconnecting the cable circuit and the openwire circuit without said voice frequency repeater apparatus having animpedance ratio equal 'to' the nominal impedance ratio between saidcircuits at'carrier frequencies and the midpoint condenser associatedtherewith being of a fixed capacity value, the auto-transformerinterconnecting the cable circuit and the open wire circuit includingsaid voice frequency repeater apparatus having an impedance ratiosmaller than the nominal impedance ratio between said circuits atcarrier frequencies and the midpoint condenser associated therewithbeing of greater capacity value than said other condenser.

In testimony whereof, I have signed my name to this specification this22nd day of December, 1926.

BENJAMIN F. LEWIS

